Multi-spectrum imaging

ABSTRACT

Multi-spectrum imaging systems and methods are provided to imaging in multiple spectrums, e.g., thermal IR (infrared) at 4 μm and 10 μm wavelengths, near-IR, and visible light, all on a same optical centerline. For example, an imaging system includes a first imager and a second imager. The first imager includes an array of thermal IR detectors, wherein the first imager is configured to receive incident photonic radiation and generate a thermal IR image, wherein each thermal IR detector comprises a photon absorber member that is configured to absorb thermal IR photonic radiation from the incident photonic radiation, and reflect remaining photonic radiation in the incident photonic radiation along an optical path of the imaging system. The second imager is disposed in said optical path of the imaging system, wherein the second imager is configured to receive the remaining photonic radiation reflected from the first imager and generate a second image.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 62/147,572, filed on Apr. 14, 2015, the disclosure of which isfully incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to imaging systems and methods and, inparticular, multi-spectrum imaging techniques.

BACKGROUND

In general, conventional imaging systems are typically designed withoptics and detectors that are configured for optimal operation in oneparticular spectral band (e.g., visible light). However, for certainapplications, it is desirable to implement an imaging system that isdesigned for multi-spectral operation in two or more discrete spectralsub-bands of the electromagnetic spectrum such as visible/near IR andmid/long wavelength IR bands. Indeed, in certain applications, theability to image a target scene in the visible and IR spectral bands canallow viewing of target objects/scenes in normal level lightingconditions as well as low-level light conditions (e.g., dusk, smoke, badweather conditions, long distance or objects that are close tobackground levels or weak emitters). There are various applications,such as military applications, where imaging targets of interest over awide range of photonic wavelengths is important or otherwise desirable.However, systems and devices for multispectral imaging applications(e.g., imaging in visible and infrared portions of the spectrum) aretypically complex and costly, due to the different optics, image sensorsand imaging electronics that are needed for each of the differentspectral bands of interest. For multispectral applications, the use ofrefractive optics is especially problematic, where refractive optics aretypically designed for specific spectral bands and cannot sufficientlyprovide wideband performance across a wide spectral range. Consequentlyfor multispectral applications, different optics must be used for eachspectral band of interest (i.e., the same refractive optics cannot becommonly used over a wide range of spectral bands).

SUMMARY

Embodiments of the invention include multi-spectrum imaging systems andmethods. For example, an embodiment of the invention includes an imagingsystem comprising a first imager and a second imager. The first imagercomprises an array of thermal IR detectors, wherein the first imager isconfigured to receive incident photonic radiation and generate a thermalIR image, wherein each thermal IR detector comprises a photon absorbermember that is configured to absorb thermal IR photonic radiation fromthe incident photonic radiation, and reflect remaining photonicradiation in the incident photonic radiation along an optical path ofthe imaging system. The second imager is disposed in the optical path ofthe imaging system, wherein the second imager is configured to receivethe remaining photonic radiation reflected from the first imager andgenerate a second image.

In another embodiment, the first imager generates a thermal IR imagefrom thermal IR photonic radiation with a wavelength of about 4 micronsor 10 microns, and the second imager comprise a visible light imager. Enyet another embodiment, the second imager comprises a near-IR imager.

Other embodiments of the invention will be described in followingdetailed description of illustrative embodiments, which is to be read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a system and method for implementingmulti-spectral imaging according to an embodiment of the invention.

FIG. 2 schematically illustrates a system and method for implementingmulti-spectral imaging according to another embodiment of the invention.

FIG. 3 schematically illustrates a thermal IR detector according to anembodiment of the invention, which can be implemented in amulti-spectral imaging system to provide thermal IR imaging, wherein thethermal IR detector is configured to further serve as a spectral filterand mirror in the multi-spectral imaging system.

FIG. 4 schematically illustrates a thermal IR detector according toanother embodiment of the invention, which can be implemented in amulti-spectral imaging system to provide thermal IR imaging, wherein thethermal IR detector is configured to further serve as a spectral filterand mirror in the multi-spectral imaging system.

DETAILED DESCRIPTION

Embodiments of the invention will now be discussed in further detailwith regard to multi-spectrum imaging systems and methods that areconfigured for imaging in multiple spectrums, e.g., thermal IR(infrared) at 4 μm and 10 μm wavelengths, near-IR, and visible light,all on a same optical centerline. It is to be understood that thevarious layers, structures, and regions shown in the accompanyingdrawings are schematic illustrations that are not drawn to scale. Inaddition, for ease of explanation, one or more structures, components,and configurations of a type commonly used to implement imaging systemsand devices may not be explicitly shown in a given drawing. This doesnot imply that any of such stuctures, components, and configurations notexplicitly shown are omitted from the actual imaging systems or devices.Moreover, the same or similar reference numbers are used throughout thedrawings to denote the same or similar features, elements, orstructures, and thus, a detailed explanation of the same or similarfeatures, elements, or structures will not be repeated for each of thedrawings. It is to be understood that the term “about” as used hereinwith regard to thicknesses, widths, percentages, ranges, etc., is meantto denote being close or approximate to, but not exactly. For example,the term “about” as used herein implies that a small margin of error ispresent, such as 1% or less than the stated amount.

FIG. 1 schematically illustrates a system and method for implementingmulti-spectral imaging according to an embodiment of the invention. Inparticular, as shown in FIG. 1, an imaging system 100 comprises anoff-axis parabolic (OAP) mirror 110, a first imager 120, a lens, and asecond imager 140. In one embodiment of the invention, the first imager120 comprises a thermal IR focal plane array having detectors that areconfigured to absorb IR photons in the thermal IR spectrum (e.g., 4 μmand/or 10 μm) and reflect radiation at other wavelengths, and generatethermal IR images. In addition, in one embodiment of the invention, thesecond imager 140 comprises a focal plane array having detectors thatare configured to detect photons in the visible and/or near-IRspectrums, and generate corresponding images.

In the system of FIG. 1, the wide spectrum primary OAP mirror 110 isused as a primary mirror for a multispectral imaging application. TheOAP mirror 110 comprises a front parabolic reflective surface 112 thatis configured to provide low loss reflection of photonic radiation overa wide spectrum as desired for the given application. The system 100receives full spectrum incident photonic radiation (1) of a given image,which enters through an aperture of the OAP mirror 110. The OAP mirror110 reflects the full spectrum of incident photonic radiation (1) fromthe reflective surface 112 and focuses the reflected incident photonicradiation (2) “off axis” to a focal point (FP1) leaving the area infront of the primary OAP mirror 110 unobstructed. While FIG. 1illustrates one exemplary embodiment in which the OAP mirror 110 is usedto focus incident photonic radiation (1), other optical elements can beused in place of the OAP mirror 110, which are capable of reflecting andfocusing a wide spectrum of photonic radiation from UV (200 nm) throughFar IR (25 μm), for example. Some examples are, but not limited to,Cassegrain mirrors, Cleartran lenses, etc.

The reflected photonic energy (2) that is focused by the primary OAPmirror 110 is directed to the first imager 120 (e.g., thermal IRimager). The first imager 120 absorbs the IR photons and creates athermal IR image. The visible and near-IR photons are not absorbed bythe first imager 120 but are reflected (3) off a surface of the firstimager 120 and directed at the re-focusing lens 130. The re-focusinglens 130 receives the reflected photonic radiation (3) and focuses thephotonic radiation to a focal point (FP2). The focused photonicradiation (4), which comprises visible and near-IR photons, is directedto the second imager 140 (e.g., visible and near-IR imager) to generatea visible light image, for example.

In the imaging system 100 of FIG. 1, the first imager 120 (e.g., thermalIR imager) serves multiple purposes. In addition to thermal IR imaging,the first imager 120 serves as a filter which separates the thermal IRradiation from the near-IR/visible radiation (by absorbing the thermalIR radiation), and further serves as a mirror which reflects theunfiltered near-IR and visible photonic radiation along the same opticalpath to one or more imagers (e.g., second imager 140). As compared toconventional imaging schemes that utilize lossy beam splitters, thereare no photonic losses by virtue of the filtering and mirroringfunctions of the first imager 120.

The imaging system 100 of FIG. 1 takes advantage of the passive IRdetector characteristics of the first imager 120 of having a surfacematerial (i.e. Cu) that will absorb photonic energy in the 3 to 15 μmspectrum (mid and far thermal IR) and pass, or reflect, the spectrum (3)below 3 μm down to 200 nm, which includes near IR, visible and UV. Inthis respect the surface of the pixel acts as a filter that absorbsthermal IR and filters UV, visible and near IR. When the pixels arearranged in an FPA (focal plane array) they create a mirror surfacesuitable to reflect enough image area (3) to be transferred to thesecondary imager 140. The system 100 is capable of imaging incidentincoming photonic scene (1) in the visible, near mid and far IR, or aquad multi-spectral imaging system. Embodiments for implementing athermal IR imager to serve as a filtering mirror will be discussed infarther detail below with reference to FIGS. 3 and 4, for example.

FIG. 2 schematically illustrates a system and method for implementingmulti-spectral imaging according to another embodiment of the invention.In particular, FIG. 2 illustrates an imaging system 200 which is similarto the imaging system 100 of FIG. 1, but wherein the imaging system 200of FIG. 2 includes a planar first surface mirror 210 interposed in theoptical path between the primary OAP mirror 110 and the first imager 120(e.g., thermal IR imager).

In the system of FIG. 2, the full spectrum of focused incident radiation(2), which is directed from the OAP mirror 110, is received by theplanar first surface mirror 210 and then reflected (2A) towards thefirst imager 120. As with the system 100 of FIG. 1, the first imager 120serves as a filter and mirror, which absorbs (filters) the thermal IRradiation from the reflected full spectrum of photonic radiation (2A)and reflects the remaining photonic radiation (3) (e.g., comprisingvisible and near-IR photonic radiation) toward the refocusing lens 130.

The mirror 210 is implemented to facilitate fitting all the systemcomponents in small or specially shaped enclosures. The second lens 130is implemented to facilitate the requirements of the distance and focallength of the second imager 140. More specifically, in the system ofFIG. 2, the total focal length from the OAP mirror 110 to the firstimager 120 is equal to A+B. At the time the photonic radiation (3)reaches the refocusing lens 130, the focal length of the incident imageis out of focus, and would generate blurred image (via the second imager140) if not refocused. In this embodiment, the refocusing lens 130 isalso used to control optical characteristics and distance from thereflecting imager 120 and the second imager 140. Here, the refocusinglens 130 is utilized to refocus the incoming photonic radiation (3) anddirect focused photonic radiation (4) towards the second imager 140. Inthe system of FIG. 1, if the first and second imagers 120 and 140 areoriented in a proper optical orientation along the optical path, therefocusing lens 130 may not be needed. However, in FIG. 2, therefocusing lens 120 will correct the focal length and focus the photonicradiation to a focal point FP2.

FIG. 3 schematically illustrates a thermal IR detector according to anembodiment of the invention, which can be implemented in amulti-spectral imaging system to provide thermal IR imaging, wherein thethermal IR detector is configured to further serve as a spectral filterand mirror in the multi-spectral imaging system. In particular, FIG. 3is a side schematic view of a thermal IR imaging device 300 according toan exemplary embodiment of the invention, which is based on acoefficient of thermal expansion (CTE) framework. The thermal IR imagingdevice 300 comprises a substrate 310 and a plurality of detectors 320formed on the substrate 310. In one embodiment of the invention, thethermal imaging device 300 can be a focal plane array that isimplemented in the multi-spectral imaging systems of FIGS. 1 and 2 asthe first imager 120 to provide thermal IR imaging, wherein eachdetector 320 corresponds to one pixel in the thermal IR focal planearray.

As shown in FIG. 3, the detector 320 comprises a resonator member 332, athermal insulating member 324, and an unpowered detector member 326. Theresonator member 322 is configured to generate an output signal having afrequency or period of oscillation. The unpowered detector member 326 isconfigured to absorb photons in the thermal IR spectrum. The unpowereddetector member 326 comprises a material having a thermal coefficient ofexpansion that causes the unpowered detector member 326 to distort(e.g., expand) due to absorption of thermal IR photons, and apply amechanical force to the resonator member due 322 as a result of thedistortion of the unpowered detector member 366 to cause a change in thefrequency or period of oscillation of the output signal generated by theresonator member 322 due to the mechanical force applied to theresonator member 3222. The thermal insulating member 324 is configuredto thermally insulate the resonator member 322 from the unpowereddetector member 326.

Although not specifically shown, the substrate 310 comprises anintegrated circuit comprising digital circuitry configured to (i)determine the frequency or period of oscillation of the output signalgenerated by the resonator member 322 as a result of the mechanicalforce applied to the resonator member 322 by the unpowered detectormember 326, and to (ii) determine an amount of said photon exposurebased on the determined frequency or period of oscillation of the outputsignal generated by the resonator member 322. The detector 320 isconnected to the digital circuitry via first and second electrodes (notshown) that are connected to the resonator member 322, and otherinterconnect structures and wiring (e.g., BEOL wiring) as may be neededfor a given layout.

In one embodiment of the invention, the unpowered detector member 326 isformed a material (or multiple materials) which can absorb photons inthe thermal IR spectrum, and which have a suitable thermal coefficientof expansion characteristic. For example, in one embodiment of theinvention, the unpowered detector member 326 is formed of copper, orother similar materials. The resonator member 322 is formed of apiezoelectric material that is configured to molecularly resonate inresponse to a drive voltage and generate an output signal having afrequency or period of oscillation, and which is configured to changethe frequency or period of the output signal generated by the resonatormember 322 in response to mechanical force exerted on the resonatormember by expansion and contraction of the unpowered detector member326. For example, in one embodiment of the invention, the resonatormember 322 is formed of AlN (aluminum nitride), or other suitablepiezoelectric materials. The thermal insulating member 324 can be formedof graphite, or other similar or suitable thermal insulating materials.

As shown in FIG. 3, a full spectrum of incident photonic radiation (30)is directed to the unpowered detector member 326 of the detector 300.The full spectrum of incident photonic radiation (30) comprises photonicradiation in the UV, visible, near-IR, mid-IR, far-IR spectrums, forexample. The full spectrum of incident photonic radiation (30) can be,for example, the focused full spectrum photonic radiation (2) reflectedfrom the OAP mirror 110 (FIG. 1) or the full spectrum photonic radiation(2A) reflected from the planar mirror 210 (FIG. 2). The unpowereddetector member 326 serves as a filter by absorbing the thermal IRradiation (mid and far IR) at, e.g., 4 and 10 microns, and serves as amirror by reflecting remaining photonic radiation (31) (e.g., remainingUV, visible and near IR radiation) off the surface of the unpowereddetector member 326, which can be redirected or used for additionalpurposes as discussed above.

FIG. 4 schematically illustrates a thermal IR detector according toanother embodiment of the invention, which can be implemented in amulti-spectral imaging system to provide thermal IR imaging, wherein thethermal IR detector is configured to further serve as a spectral filterand mirror in the multi-spectral imaging system. In particular, FIG. 4is a side schematic view of a thermal IR imaging device 400 according toan exemplary embodiment of the invention, which has a plurality ofdetectors 320 which are similar in structure and function to thedetector 320 discussed above with reference to FIG. 3, but wherein eachdetector 320 is formed on wedge structure 328 to disposed each detector320 at some angle from the plane of the substrate 310.

With the thermal IR imagine device 400 of FIG. 4, the plane of thesubstrate 310 can be disposed substantially perpendicular to an “opticalcenterline” of incoming incident photonic radiation 30, while allowingthe reflected/filtered photonic radiation 31 to be directed away fromeach detector 320 at some reflection angle, which depends, in part, onthe angle of the wedge structures 328, and directed towards some otherimaging device or mirror in along an optical path of the multi-spectralimaging system. For example, the thermal IR imaging device 400 of FIG. 4can be used to implement the first imager 120 in FIG. 1, such that thesubstrate plane of the first imager 120 can be positioned perpendicularto the optical centerline of the focused photonic radiation (2) directedfrom the OAP mirror 110, which still being able to directreflected/filtered photonic radiation (30) towards the lens 130.Furthermore, the implementation of the angled pixel structure in FIG. 4serves to accommodate the reflective characteristics of the absorbersurface 326 which have been enhanced for better performance of thethermal IR with a textured surface or other coating materials.

In another embodiment the thermal IR imaging device 400 of FIG. 4 can beused in place of the OAP mirror 110 in FIGS. 1 and 2, wherein thesubstrate plane of the thermal IR imaging device 400 is positionedperpendicular to the optical centerline of focused light that isdirected to the thermal IR imaging device 400 from a multi-spectral lensdevice that focuses incoming incident photonic radiation of an imagefrom a scene. In the configuration, the thermal IR imaging device 400reflects filtered photonic radiation “off axis” to downstream components(e.g., mirror, imager, etc.) of the multi-spectral imaging systemwithout interference from the incoming incident photonic radiation

The OAP mirror 110 in FIGS. 1 and 2, for example, can be fabricatedusing the materials and methods disclosed in U.S. patent applicationSer. No. 14/315,270, filed on Jun. 25, 2014, entitled “Wide SpectrumOptical Systems and Devices Implementing First Surface Mirrors,” toimplement low cost first surface mirrors with wide spectrum performance.Moreover, it is to be understood that other thermal IR detectorconfigurations can be implemented in the multispectral imaging systemsas shown in FIGS. 1 and 2, and that the detector structures 320 shown inFIGS. 3 and 4 are just example embodiments. For example multispectralimaging systems according to embodiments of the invention can beimplemented using various types of passive CTE-based photon detectorstructures as disclosed, for example, in U.S. patent application Ser.No. 15/099,440, filed on Apr. 14, 2016, U.S. Pat. No. 9,012,845, U.S.Provisional Application Ser. No. 62/206,798, filed on Aug. 18, 2015, andU.S. Provisional Application Ser. No 62/148,829, filed on Apr. 17, 2015,all of which are incorporated herein by reference. The passive detectorstructures and associated pixel circuity disclosed in these patents canbe used to implement thermal IR detectors (e.g., detectors 120 and 320)and associated pixel circuitry in the imaging systems disclosed herein.

Although exemplary embodiments of the invention have been describedherein with reference to the accompanying drawings, it is to beunderstood that the scope of the invention is not limited to thoseprecise embodiments, and that various other changes and modificationsmay be affected therein by one skilled in the art without departing fromthe scope or spirit of the invention.

What is claimed is:
 1. An imaging system, comprising: a first imagercomprising an array of thermal infrared (IR) detectors, wherein thefirst imager is configured to receive incident photonic radiation andgenerate a thermal IR image, wherein each thermal IR detector comprisesa photon absorber member that is configured to absorb thermal IRphotonic radiation from the incident photonic radiation and convert theabsorbed thermal IR photonic radiation to thermal energy, and reflectremaining photonic radiation in the incident photonic radiation along anoptical path of the imaging system, wherein the photon absorber memberswithin the array of thermal IR detectors collectively form a reflectingsurface; and a second imager disposed in said optical path of theimaging system, wherein the second imager is configured to receive theremaining photonic radiation reflected from the reflective surfacecollectively formed by the photon absorber members within the array ofthermal IR detectors of the first imager and generate a second image;wherein the first imager comprises: a substrate, wherein each thermal IRdetector is formed on the substrate; and wherein each thermal IRdetector further comprises: a resonator member configured to generate anoutput signal having a frequency or period of oscillation; and whereinthe photon absorber member comprises an unpowered detector member thatis configured for photon exposure, wherein the unpowered detector membercomprises a material having a thermal coefficient of expansion thatcauses the unpowered detector member to distort due to said photonexposure, wherein the unpowered detector member is further configured toapply a mechanical force to the resonator member due to said distortionof the unpowered detector member, and cause a change in the frequency orperiod of oscillation of the output signal generated by the resonatormember due to said mechanical force applied to the resonator member; anda thermal insulating member configured to thermally insulate theresonator member from the unpowered detector member; and digitalcircuitry configured to (i) determine the frequency or period ofoscillation of the output signal generated by the resonator member as aresult of the mechanical force applied to the resonator member by theunpowered detector member, and to (ii) determine an amount of saidphoton exposure based on the determined frequency or period ofoscillation of the output signal generated by the resonator member. 2.The imaging system of claim 1, wherein the first imager generates athermal IR image from thermal IR photonic radiation with a wavelength ofabout 4 microns or 10 microns.
 3. The imaging system of claim 1, whereinthe second imager comprise a visible light imager.
 4. The imaging systemof claim 1, wherein the second imager comprises a near-IR imager.
 5. Theimaging system of claim 1, further comprising an off-axis parabolicmirror configured to focus the incident photonic radiation received bythe first imager.
 6. The imaging system of claim 5, further comprising aplanar mirror disposed in said optical path between the off-axisparabolic mirror and the first imager.
 7. The imaging system of claim 5,further comprising a lens disposed in said optical path between thefirst imager and the second imager, wherein the lens is configured tofocus the remaining photonic radiation reflected from the first imager.8. The imaging system of claim 1, wherein each thermal IR detectorcomprises a wedge structure that maintains the thermal IR detector at anangle with regard to a plane of the substrate.